Various types of coatings were conventionally used to improve the cutting performance and extend the life of cutting tools. In order to improve the performance of coatings, multi-layer coatings stacked with multiple layers were used, wherein each layer has a thickness of few nanometers. In such multi-layer coatings, the compositions of adjacent layers were configured differently, thereby resulting in different lattice parameters and interaction between adjacent layers. Thus, the hardness and wear resistance of the multi-layer coatings were improved. However, when multiple layers with only a few nanometers of thickness were stacked, there was a problem in that the accumulated torsion stress from the stacked structure caused a decrease in impact-resistance, hence increasing the occurrences of brittle fractures.
In another prior art technology, the toughness and impact-resistance of a multi-layer coating were enhanced by employing an interlaid thick layer, which has a thickness ranging from a few hundred nanometers to a few micrometers, into a structure in which multiple layers were deposited, wherein each multiple layer has a thickness of a few nanometers. The thick layer lowered the high torsion stress caused by the deposited layers, wherein each layer has a few nanometers thickness to improve the toughness and impact-resistance of the multi-layer coating. However, in order to achieve the above, the interlaid layer had to be thick, which consequently lowered the hardness enhancement effect expected by the interaction between the layers with a thickness of few nanometers. This causes a problem of degrading the hardness and wear-resistance of the multi-layer coating.
Thus, the conventional multi-layer coatings could only improve one of the mechanical properties, i.e., hardness or toughness. Accordingly, cutting tools having the multi-layer coatings of the prior art were only limited to achieving one purpose, i.e., high wear-resistance or high impact-resistance. Moreover, since one of the mechanical properties (i.e., either wear-resistance or impact resistance) was relatively inferior compared to the other property, the multi-layer coating of the prior art had limitations in extending the lifespan of the cutting tool.